1. Field of the Invention
The present invention relates to a battery control apparatus for a hybrid powered vehicle, comprising a battery set having a predetermined number of battery blocks connected in series, wherein the control apparatus is for maintaining the SOC (state of charge) of the battery at a target value using a generator driven by an engine.
2. Description of the Related Art
Hybrid vehicles provided with an engine-driven generator in addition to a motor for driving the vehicle are known. In such hybrid vehicles, a battery is provided, electricity from the battery is used to drive the drive motor, and the battery is charged with electricity from the generator. The drivemotor and the engine rotate the wheels to propel the vehicle. During normal travelling, the driving of the motor and of the generator are controlled such that the SOC of the vehicle-mounted battery is maintained close to a predetermined target value (for example, approximately 50%).
However, when such a vehicle travels under strenuous conditions, such as climbing a long slope, discharge from the battery may continue for a prolonged period of a time. In such cases, the SOC of the battery progressively decreases, possibly resulting in a drastic lowering of the battery SOC. For this reason, limit values of SOC are normally set, with the lower limit value at approximately 20% and the upper limit value at approximately 80%. For example, discharge from the battery may be inhibited when the SOC decreases below 20%, and charging of the battery inhibited when the SOC increases above 80%. The battery SOC is thus maintained at a level between 20% to 80%.
The vehicle-mounted battery referred to here is a type having an output direct current voltage of approximately 250V. Accordingly, this vehicle-mounted battery comprises a plurality of battery cells connected in series forming a battery set. Typically, a plurality of battery cells are serially connected to form a battery block, and a plurality of these battery blocks are serially connected to constitute a battery set. For example, 12 cells form one block, and 20 blocks constitute one battery set.
An SOC of a battery is typically determined by accumulation of charge and discharge amount. In a hybrid vehicle wherein the SOC is controlled to be maintained at approximately 50%, if any difference exists between the determined charge and discharge amount and the actual charge and discharge amount, the error tends to be amplified by the accumulation process over a long period of time. In addition, as the SOC of a battery set is calculated from the total charge and discharge amount of the entire set, the SOC of each battery cell may differ from the determined SOC. The operating conditions, such as temperature, are not necessarily identical for each cell, and variation in SOC values between each cell can not be prevented.
Accordingly, there may exist cases in which, while the SOC calculated from the charge and discharge amount has not decreased below 20%, the SOC of a particular cell within the battery has reached 0%. When using a battery set, discharge is continued from other battery cells even after the SOC of one battery cell has become 0%. Consequently, a discharge current continues to flow through the 0% SOC battery cell (overdischarged cell), which may cause generation of hydrogen gas in that cell.
In order to solve such problems, the inventor of the present invention proposed, in Japanese Patent Application No. Hei 9-338436, a system where 0% SOC in a battery cell is detected through determination of a voltage difference between battery cells. This technology is described below. It should be noted that the content of Japanese Patent Application No. Hei 9-338436 is not yet publicly disclosed at the time of the filing of the present application.
In a battery set comprising 200 or more battery cells, it is difficult to detect a 0% SOC in one cell from the voltage of the entire battery set. However, by determining the voltage value for each battery block comprising approximately 10battery cells, and comparing these values, a difference of approximately 1can be detected in a battery block containing a 0% SOC battery cell in comparison with other battery blocks.
More specifically, in normal usage status, the voltage levels of each battery block are approximately equal to each other because no 0% SOC battery cell exists. When discharge proceeds and the SOC of one battery cell becomes 0%, the voltage of this cell becomes 0V. As a result of the voltage of one cell being 0V, the voltage of the battery block containing this 0% SOC cell suddenly decreases by 1V or more. The occurrence of a 0% SOC battery cell can therefore be detected through the detection of a voltage difference of 1V or more between battery blocks.
In the invention described in Japanese Patent Application No. Hei 9-338436, output power is restricted when such 0% SOC battery cell is detected. When discharge further continues in spite of the output power restriction, the battery is disconnected and no further discharge is permitted at all.
However, the output power restriction performed in this case is a control operation for increasing the amount of charging, wherein the torque output is maintained in proportion to the amount of accelerator stepping. It was therefore possible that discharge further continues, thereby continuing the generation of hydrogen gas in the battery cell.